1. Field of the Invention
The present invention relates to an control apparatus for an antilock brake system (ABS). More particularly, it relates to an ABS control apparatus wherein a braking hydraulic pressure is controlled to an appropriate value in accordance with the detection of the wheel slip of a four-wheel drive vehicle.
2. Description of the Prior Art
Heretofore, engine braking and pumping braking have been utilized in the descent of a motor vehicle along a steep slope. Herein, when the speed of the motor vehicle has somewhat risen into a region permitting an antilock brake system (ABS) control operation, the drive thereof enters an ABS control. Therefore, the stopping distance of the motor vehicle sometimes lengthens more than in the case of locking wheels. This is a phenomenon similar to the fact that, when the motor vehicle is to stop on a gravel road or the like, the stopping distance becomes shorter under the case of locking the wheels than under the ABS control operation, owing to the subsidence of the wheels into gravel.
In this regard, as techniques for preventing such an excessively sensitive control in the descent of the motor vehicle along the slope under the ABS control, by way of example, the official gazette of Japanese Patent Application Laid-open No. 503525/1994 discloses an antilock brake system (ABS) control apparatus wherein the sensitivity of the ABS control is altered upon the detection of a braking state during the descent along the slope.
The control apparatus disclosed in the aforementioned official gazette, however, has the problem that the control sensitivity cannot be altered unless the ABS control is once started, so the braking distance of the motor vehicle becomes longer owing to delay of the alteration of the control sensitivity.